The present invention relates to a pneumatic tire, more particularly to a tread-reinforcing band formed by spirally winding a specially treated organic fiber cord.
In order to improve the high-speed durability of a pneumatic tire such as passenger car tire, motorcycle tire and the like, a tread reinforcing band made of a spirally wound cord is widely used in the tread portion alone or in combination with a breaker.
Usually, nylon fiber cords are used as the cords of such a tread reinforcing band because of the following reason. During vulcanizing a green tire in a mold, the inside of the green tire is pressurized by the use of an inflatable bladder to press the outer surface of the green tire onto the inner surface of the mold. Therefore, if the band is not stretched or the amount of stretch is insufficient, the tread rubber is not fully pressed against the inner surface of the mold. As a result, the tire uniformity is deteriorated and vibrations are liable to occur during running especially high speed running. Further, in the worst case, molding defects occur in the tread portion. Accordingly, in order to secure a sufficient stretch during vulcanization, nylon fiber cords which are relatively low in the modulus and thus stretchable during vulcanization are widely used.
On the other hand, pneumatic tires are nowadays in urgent need of effective weight reductions to improve fuel economy or to reduce carbon dioxide emissions.
In order to reduce the weight of a tread reinforcing band, if the low modulus nylon fiber cords used in a band is decreased in the number and/or thickness, then it becomes difficult to secure the necessary strength, especially the high-speed durability of the tire.
If high-strength cords such as aromatic polyamide fiber cords are used as the band cords, the weight of the band can be reduced, but the above-mentioned problems arise since the conventional high-strength cords such as aromatic polyamide fiber cords have very high moduli and are hard to stretch during vulcanization as well as during use in the vulcanized tire.